How flip model upside down?
You should be able to achieve this by using a custom transform orientation.



With your object selected, tab into Edit Mode. Select an edge that you want to scale along.

For example: 



Then, still in Edit Mode, click the [+] symbol under Transform Orientations.



This adds the selected edge as a new Orientation.

It is handy to give it a name under the tool shelf on the left side.



Now you will see this:

 and 

Finally, select all, hit S to scale and then TWICE the axis the points in the same direction as the edge you selected and -1 on your keyboard.

In my case it is the y-Axis.

So I would hit S,y,y,-,1.

Before:

 

After:



On the bottom left you'll see:



Unfortunately, that will mess with your Normals. To solve that, simply hit Ctrl + N. In my case, I had to manually select and flip the Normals inside the B's arcs.

Not a big problem, but if anyone knows how to get around this, please comment.

Result: